SINGULART - World's Leading Online Art Gallery
WikiArt - The Encyclopedia of Fine Art
Saatchi Art - Online art platform for buying, selling and discovering art.
Artspace - Art platform to discover and buy contemporary art.
ArtMajeur - +1 Million original artworks, Free Shipping Worldwide, Low prices guaranteed, 14 days return: buy all the art you love on Artmajeur!
Google Art Project - Chrome extension from the Google Cultural Institute